Method for Ultra-Fast Controlling of a Magnetic Cell and Related Devices

ABSTRACT

The present invention relates to a device and corresponding method for ultrafast controlling of the magnetization of a magnetic element. A device ( 100 ) includes a surface acoustic wave generating means ( 102 ), a transport layer ( 104 ), which is typically functionally and partially structurally comprised in said SAW generating means ( 102 ), and at least one ferromagnetic element ( 106 ). A surface acoustic wave is generated and propagates in a transport layer ( 104 ) which typically consists of a piezo-electric material. Thus, strain is induced in the transport layer ( 104 ) and in the ferromagnetic element ( 106 ) in contact with this transport layer ( 104 ). Due to magneto elastic coupling this generates an effective magnetic field in the ferromagnetic element ( 106 ). If the surface acoustic wave has a frequency substantially close to the ferromagnetic resonance (FMR) frequency ν FMR  the ferromagnetic element ( 106 ) is absorbed well and the magnetization state of the element can be controlled with this FMR frequency. The device can be used in an RF-magnetic resonator, a sensor and a camera. The corresponding method can be used for ultrafast reading-out and switching of magnetic components and in magnetic logic.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of its parent application, U.S. patentapplication Ser. No. 10/584,699, entitled “Method for ultra-fastcontrolling of a magnetic cell and related devices,” which issued asU.S. Pat. No. 7,638,922 on Dec. 29, 2009. The parent application was theU.S. national phase of international application PCT/EP04/014815, filedDec. 24, 2004. The parent application and the international applicationclaim the priority of U.S. Provisional Application 60/533,323, filedDec. 29, 2003 and of European application 03447312.4, filed Dec. 24,2003. This application claims the priority of these prior applications.The disclosures of these prior applications are incorporated byreference into this application.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of magnetics. The inventionrelates to methods, techniques and corresponding devices for controlledmanipulation of magnetisation states of ferromagnetic entities as e.g.magnetic layers, cells and components. The invention also relates to RFresonators and magnetic logic and magnetic memories.

BACKGROUND OF THE INVENTION

Currently, magnetisation states in a ferromagnetic component, as e.g. aferromagnetic MRAM cell, are being manipulated, e.g. switched orchanged, and assessed, e.g. read or written, by magnetic fieldsgenerated by neighbouring electrical currents, or by applying externalmagnetic fields.

The technique of magnetic field induced switching by current conductorsis widespread and is currently used in a wide series of commercialproducts. Several types of magnetic field induced switching by currentconductors are known. The switching is generally done by a staticmethod, where currents high enough to switch the element are applied andthe element switches after waiting long enough. An alternative methodfor driving magnetisation read-out or for changing the magnetisationstate of a ferromagnetic component is making use of ferromagneticresonance (FMR). Ferromagnetic resonance is an intensively studiedphenomenon, which is well known, and its use for the switching andassessment of ferromagnetic components offers several speed and poweradvantages as compared to regular methods. The mechanism known in theart as ‘precessional switching’ is based on the ferromagnetic resonanceproperties of the magnetic device and allows magnetisation reversal withless power and at higher frequencies than with other, older switchingschemes.

All the above described techniques, however, have several differentproblems, such as e.g. current lines are needed for both biasing andmagnetic assessment, a bit selection scheme has stringent timingrequirements, power consumption is relatively high and differentmetallisation levels are required. Furthermore, reference cells can benecessary for comparing states during read-out, which reduces theeffective cell density. Typically one reference bit per data storage bitis used.

Operating at ferromagnetic resonance frequencies leads to difficultiesin controllability and integration. Moreover there is the constant needfor external magnetic fields to control the magnetic properties, whichlimits the use of magnetic materials, even at low frequencies, due tofield spreading and power consumption. The latter makes it hard to useFMR in several applications.

It is furthermore a known characteristic of magnetic materials thattheir magnetic state can be altered by the presence of stress and/orstrain in a magnetic material. A typical suitable material for stressstate alteration is Ni which is described e.g. in Sander D., “Thecorrelation between mechanical stress and magnetic anisotropy inultrathin films”, Reports on Progress in Physics 62, (1999) p 809.Typically, stress is induced by applying a voltage to a piezo-electricmaterial and the use of stress is only known to be controllable at lowfrequencies. This limits the use of stress induced switching in e.g.ferromagnetic memory cells.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide novel methods andcorresponding devices for ultra fast assessment of magnetic elementswith high controllability.

It is a further object of the present invention to provide a read-outscheme for exchange biased spin-valve or tunnel-junction structures, ascan be used in e.g. an MRAM-like structure, working at frequencieshigher than 1 GHz.

It is furthermore an object of the present invention to provide anin-plane magnetic sensor working at frequencies higher than 1 GHz.Furthermore it is an object of the present invention to provide anin-plane magnetic camera working also at frequencies higher than 1 GHz.

It is another object of the present invention to provide a switchingscheme for magnetic layers and components.

It is furthermore an object of the present invention to provide an RFmechanical resonator.

It is another object of the present invention to provide a novel drivingmechanism in magnetic logic.

It is furthermore another object of the present invention to provide amethod for active compensation of changes in switching behaviour of amagnetic switch.

The above objectives are accomplished by methods and devices accordingto the present invention.

The present invention provides a method of using magneto-elastic energyconversion to determine or identify or change the magnetisation state ofa ferromagnetic element. The use of magneto-elastic energy conversioncan be between a magnetic element and a SAW in a piezoelectric layer tointeract with the magnetisation state of the magnetic element. Thepresent invention relates to an electronic device comprising apiezoelectric layer and a magnetic element and means for magneto-elasticenergy conversion between the magnetic element and a SAW in thepiezoelectric layer to interact with the magnetisation state of themagnetic element.

The present invention relates to a device allowing magnetic propertyinteraction comprising at least one surface acoustic wave generatingmeans, a transport layer, and at least one ferromagnetic element, havinga ferromagnetic resonance frequency ν_(FMR), wherein the surfaceacoustic wave generating means is adjusted to generate in the transportlayer a surface acoustic wave having a wavelength λ_(SAW) and having afrequency ν_(SAW) substantially equal to said ferromagnetic resonancefrequency ν_(FMR) or an integer multiple of said ferromagnetic resonancefrequency ν_(FMR).

The present invention also relates to a device allowing magneticproperty interaction, the device comprising a layer comprisingpiezoelectric material, said layer being adapted for transporting asurface acoustic wave having a frequency ν_(SAW), and at least oneferromagnetic element, having a ferromagnetic resonance frequencyν_(FMR) and being capable of magneto-elastic energy conversion, whereinsaid surface acoustic wave frequency ν_(SAW) is substantially equal tosaid ferromagnetic resonance frequency ν_(FMR) or an integer multiple ofsaid ferromagnetic resonance frequency ν_(FMR) such that said surfaceacoustic wave interacts with said at least one ferromagnetic element toinfluence a magnetisation state of said ferromagnetic element. Said atleast one ferromagnetic element may be magneto-strictive. Themagnetisation state may be used as output of the device. Said integermay be an even integer number, such as e.g. 2. The device may compriseat least one surface acoustic wave generating means for generating saidsurface acoustic wave having a frequency ν_(SAW). The layer comprisingpiezoelectric material may be a transporting layer. The magnetostrictive material is a means for magneto-elastic energy conversion.Said frequency ν_(SAW) may be in a range having a width corresponding toa certain fraction of a width of an absorption peak corresponding withsaid ferromagnetic resonance frequency value ν_(FMR) or an integermultiple thereof, and which is centred around the ferromagneticresonance frequency value ν_(FMR) or around an integer multiple thereof,said fraction being 100%, preferably 50%, more preferably 25%, even morepreferably 10%, still more preferably 2% or even still more preferably1%. Said ferromagnetic element may be in contact with said layercomprising piezoelectric material or with said surface acoustic wavegenerating means. Said contact may be direct contact. Said ferromagneticelement may be not in direct contact with said surface acoustic wavegenerating means. Said ferromagnetic element may be a part of saidsurface acoustic wave generating means. The device allowing magneticproperty interaction may comprise a means for reading out the magneticstate of the at least one ferromagnetic element. This means for readingout may be a system that can sense the magnetic response to the surfaceacoustic wave. This means for reading out may be a system for performinga resistance measurement. The magnetization state may be the readingout, also referred to as output, of the system. The reading out may beinterpreted. Different effects may be used as an output, like mostcommonly magnetoresistance, in components as tunnel junctions and spinvalves, as well as in single layers as AMR. Furthermore all techniquesbased on or derived from magnetic imaging techniques may be seen as anoutput, like, without wanting to be exhaustive, Magnetic ForceMicroscopy, (Scanning) Hall Probe Microscopy, Magneto Optic Kerr Effectmeasurements, . . . Also inductance based techniques can be used withpick-up coils, field sensors (read head, Hall probe), . . . or e.g.measurements of transfer characteristics of pulse lines, since thesesense neighbouring magnetic films and their characteristics.

The frequency of the surface acoustic waves may be chosen in a narrowfrequency range around the ferromagnetic resonance frequency or multiplethereof.

This range is a material property and depends on the absorptioncoefficient which for the purpose of this description may be defined asthe width of the absorption peak, of the magnetic material. The bordersof the peak which define the width, are situated where the absorption isreduced to substantially 0, which may e.g. 0.1, more preferably 0.01,even more preferably 0.001, still more preferably 0.0001. Otherparameters which could be used to represent the width of the absorptionpeak may be the damping parameter a or the Full Width at Half Max value(FWHM). The absorption peak may preferably be as narrow as possible. Thefrequency range preferably used may then be the range with a widthcorresponding to a certain fraction of the width of the absorption peak,and which is centred around the maximum absorption frequency value. Thisfraction may for example be 100%, preferably 50%, more preferably 25%,even more preferably 10%, still more preferably 2% or even still morepreferably 1%.

The SAW may alter the properties of the magnetic element, such that theaction of an additionally applied magnetic field will be different thanwithout the SAW. The presence of such an additional magnetic field maythus be advantageous for certain embodiments of the present invention.

The device may comprise a means for generating such an additionalmagnetic field at the ferromagnetic resonance frequency or an integermultiple of said ferromagnetic resonance frequency. The device may beused for ‘parametric amplification’ where oscillations induced by a weakmagnetic field force coming from an additional magnetic field can beamplified by the parametric mechanism. The SAW may act as a parametricpump at double or 2n (an even number of) Times—n being an integernumber, advantageously a small integer number as for instance but notlimited to 1, 2, 4, 6, 8—the frequency of an oscillating system drivenby the weak additional magnetic field force, and, depending on the phasesynchronization between the driving force and the parametric pump, anamplification of the oscillation amplitude, indicated by e.g. anabsorption peak, or an active compensation or enhancement of thedamping, related to the width of the absorption, may be achieved.

The frequency of the surface acoustic wave thus may be such that it isabsorbed significantly by the ferromagnetic element. The absorption ofthe surface acoustic wave may be at least 1%, preferably 25%, morepreferably 50%, even more preferably 75% and most preferably at least99% of the absorption at the ferromagnetic resonance frequency.Furthermore, the surface acoustic wave generating means may comprisepart of said layer comprising piezoelectric material. The propagatedsurface acoustic wave may create an effective magnetic field due tomagneto-elastic energy conversion g in said ferromagnetic element so asto manipulate or affect or change a magnetic property of saidferromagnetic element. The magnetic property may be a magnetisationstate of said ferromagnetic element. Said magnetic property may be anyof switching behaviour, coercivity, biasing, permeability,susceptibility, damping behaviour or absorption behaviour. The length ofthe ferromagnetic element may be smaller than the wavelength of thesurface acoustic wave λ_(SAW), preferably smaller than a quarter of thewavelength of the surface acoustic wave λ_(SAW). The length may belarger than the wavelength of the surface acoustic wave λ_(SAW). Thewidth of the ferromagnetic element may be smaller than the wavelength ofthe surface acoustic wave λ_(SAW), preferably smaller than a quarter ofthe wavelength of the surface acoustic wave λ_(SAW). The width also maybe larger than the wavelength of the surface acoustic wave λ_(SAW). Theferromagnetic element may be a functional or structural part of amagnetic component. This magnetic component may be any magnetoresistivedevice, such as for example an AMR, a TMR or a GMR device. The magneticcomponent may be a spin valve without pinned layers. The magneticcomponent may be for example a spin valve or a tunnel junction, whichmay comprise a reference layer with a pinned magnetisation. The surfaceacoustic wave used in the device may be any of a shear wave and aRayleigh wave. It also may be any other suitable surface acoustic wave.The ferromagnetic element may be oriented such that the angle betweenthe direction of an easy axis of the ferromagnetic element and thedirection of the induced effective magnetic field is different from 0°,preferably is larger than 45°, more preferably is larger than 80°, mostpreferably is 90°. The surface acoustic wave generating means may be orcomprise at least one InterDigitated Transducer. Furthermore, additionalsurface acoustic wave generating means may be included. E.g. the devicemay comprise a second acoustic wave generating means. The first surfaceacoustic wave generating means may be for generating a shear wave in afirst surface acoustic wave propagation direction and the second surfaceacoustic wave generating means may be for generating Rayleigh waves in asecond surface acoustic wave propagation direction. The first surfaceacoustic wave propagation direction and said second surface acousticwave propagation direction may be orthogonal on each other. The devicemay also have for at least one surface acoustic wave (SAW) generatingmeans a surface acoustic wave detection means positioned opposed to theSAW generating means relatively to the ferromagnetic element. Thissurface acoustic wave detection means may be placed diametricallyopposed to the SAW generating means relatively to the ferromagneticelement. In this device the SAW generating means may be combined withother means of assessing magnetic elements, i.e. more conventional wayssuch as magnetic fields generated by current conductors close by, orthermally or spin torque (angular momentum) induced magnetic changesgenerated by heaters, laser pulses, or currents through the element.

The device may also comprise a plurality of ferromagnetic elementsordered on top of said layer comprising piezoelectric material, so as toprovide a magnetic image. The plurality of ferromagnetic elements may beordered on top of said surface acoustic wave generating means. Thedevice then can act as a magnetic camera. The ferromagnetic elements maybe ordered in a number of rows and columns.

The invention also relates to a method for sensing an environmentalparameter, said method comprising the steps of allowing at least oneferromagnetic element to interact with an environment of which aenvironmental quantity has to be measured, generating a surface acousticwave in a layer comprising piezoelectric material, said surface acousticwave interacting with said at least one ferromagnetic element,dynamically measuring the variation of a characteristic parameterinfluenced by said ferromagnetic element, deriving from said variationof a characteristic parameter a corresponding value of a physicalquantity of said ferromagnetic element. Said physical quantity of saidferromagnetic element may be a magneto resistance of said ferromagneticelement. Said physical parameter may be a magneto resistance of saidferromagnetic element. In this method, said deriving from said variationof a characteristic parameter a corresponding value of said physicalquantity may comprise the steps of deriving from the dynamic measurementa degree of anisotropy of said at least one ferromagnetic element andderiving from said degree of anisotropy a corresponding value of saidquantity. The method of sensing may be applied in any of the abovedescribed devices. In this method the SAW generating means may becombined with other means of assessing magnetic elements, i.e. moreconventional ways such as magnetic fields generated by currentconductors close by, or thermally or spin torque (angular momentum)induced magnetic changes generated by heaters, laser pulses, or currentsthrough the element. Furthermore, said quantity may be anelectromagnetic field, a temperature, a pressure, a density or a stressor other physical property. The variation in magneto-resistance of saidat least one ferromagnetic element may be induced by the magnetisationor magnetisation direction of said ferromagnetic element.

The invention may also relate to a method for creating a magnetic image,comprising the steps of allowing a plurality of ordered ferromagneticelements to interact with an environment of which an image is to becreated, generating a surface acoustic wave in a layer comprisingpiezoelectric material, said surface acoustic wave thus interacting withsaid plurality of ordered ferromagnetic elements, dynamically measuring,at least a number of, preferably for each of, said plurality offerromagnetic elements the variation of characteristic parametersinfluenced by said ferromagnetic elements and deriving from saidvariation of said characteristic parameters a corresponding value of aphysical quantity for said number of said plurality of ferromagneticelements. The physical quantity of said number of said plurality offerromagnetic elements may be a magneto resistance of said ferromagneticelements. In the method, said allowing the plurality of orderedferromagnetic elements to interact with an environment and saidgenerating a surface acoustic wave may be performed one time for allferromagnetic elements in parallel and said dynamically measuring thevariation and said deriving a corresponding value may be performed on aferromagnetic element basis. The method may be applied to the camera asdescribed above.

The invention also relates to a method for reading out a readout-valuefrom a device as, for example, described above, comprising the steps ofgenerating a surface acoustic wave, such that a precessional movement ofthe magnetisation in said at least one ferromagnetic element is achievedand such that a corresponding magnetisation state of said at least oneferromagnetic element is not switched, dynamically measuring thevariation of a characteristic parameter influenced by said element andderiving from said variation of said characteristic parameter saidread-out value. The characteristic parameter influenced by saidferromagnetic element may be a magneto resistance of said ferromagneticelement. In the method, said deriving from said variation of saidcharacteristic parameter said read-out value may be deriving a phasedifference between the input signal applied to said surface acousticwave generating means and the output signal obtained from said dynamicmeasurement of said characteristic parameter and deriving from saidphase difference a read-out value. Said corresponding magnetisationstate is the state corresponding with the magnetisation in said at leastone ferromagnetic element. The method may be applied to any of thedevices as described above. In the method, the read-out value cancorrespond to only a number of distinct specific values. The number ofdistinct values may be two and the values can be represented as ‘1 ’ and‘0’.

The invention also may relate to a method for switching at least oneferromagnetic element, comprising the step of generating a surfaceacoustic wave, for achieving a precessional movement of themagnetisation in said ferromagnetic element and orienting acorresponding magnetisation state of said ferromagnetic element. Thecorresponding magnetisation state is the state corresponding with themagnetisation in said ferromagnetic element.

The orienting of the magnetisation state of the ferromagnetic elementmay be performed by generating a ferromagnetic element specificadditional field. This additional field may be quasistatic, pulsed or atfrequencies equal to the ferromagnetic resonance ν_(FMR) or an integermultiple of this frequency ν_(FMR). The additional field may be providedby any means that changes the effective magnetic field felt by themagnetic element. The SAW field and the additional field may also betuned (e.g. timed) such that only for a certain time or phasesynchronization the switching can be initiated. When an array ofmagnetic components needs to be addressed, the synchronization as wellas the amplitude of both the SAW generating means and the additionalfield may be used to provide selection of a single magnetic element outof the array for reading or writing.

The surface acoustic wave may be a Rayleigh wave and the angle betweenan easy axis of the ferromagnetic element and the direction of theeffective field may be different from 0° and may preferably be more then45°, more preferably more then 80° and most preferably may be 90° duringthe first half period of the Rayleigh wave.

The surface acoustic wave also may be a shear wave and the angle betweenthe direction of an easy axis of said ferromagnetic element and thedirection of the effective magnetic field generated by said device maypreferably be larger than 45°, more preferably larger than 80° and mostpreferably may be 90°.

The invention also may relate to a method for combined reading andwriting of at least one ferromagnetic element. The device then has twosurface acoustic wave generating means, whereby the first surfaceacoustic wave generating means is used for switching according to any ofthe methods for switching as described above and the second surfaceacoustic wave generating means may be used for sensing or readingaccording to any of the methods for reading or sensing as describedabove. An additional magnetic field at the ferromagnetic resonancefrequency or an integer multiple of said ferromagnetic resonancefrequency may be applied. The surface acoustic wave may have a frequencythat is substantially equal to the ferromagnetic resonance frequency oran integer multiple thereof. The integer multiple may be an even integermultiple, such as e.g. 2.

The invention also may relate to a magnetic resonator comprising adevice as described above and furthermore being equipped with a tip thatis made of magnetic material and that is supported by a cantilever-typestructure and furthermore being positioned near the ferromagneticelement of said device. The tip then senses the GHz frequencyoscillation of the magnetic effective field and a corresponding signalcan be outputted.

The invention may also relate to an oscillator, a resonator or a filteras is used in telecommunications that is made out of a ferromagneticmaterial or ferromagnetic multi layer stack on top of a piezoelectriclayer, where the ferromagnetic resonance properties (absorptionfrequency or width of the absorption peak) are altered by the SAW,according to the devices as described above.

The invention also relates to the use of a device as described above foruse in magnetic logic. The application of a surface acoustic wave may bethe driver of a magnetic logic since it can decrease the thresholdenergy for magnetic data transport.

The invention also may relate to a method for active tuning of a workingfrequency of a surface acoustic wave, said method comprising the stepsof monitoring the absorption of a surface acoustic wave, generated by asurface acoustic wave generating means, by the ferromagnetic element,deriving from said absorption characteristics the difference between theworking frequency of the surface acoustic wave and the ferromagneticresonance frequency of said ferromagnetic element, and tuning theworking frequency of the surface acoustic wave generating means towardsthe ferromagnetic resonance frequency. The method may be performed in asystem comprising a surface acoustic wave detection means and may beperformed in a system comprising any of the devices as described above.Furthermore, said tuning of the working frequency of the surfaceacoustic wave generating means towards the ferromagnetic resonancefrequency may be tuning the working frequency to a frequency slightlydifferent from the ferromagnetic resonance frequency. Furthermore, thefrequency may correspond with an absorption of said surface acousticwave by said ferromagnetic element within 1% and 99%, preferably 50% and90%, more preferably 70% and 90% of the absorption of said surfaceacoustic wave by said ferromagnetic element at the ferromagneticresonance frequency.

The invention also relate to an oscillator, which may e.g. be, aresonator or a filter as is used in telecommunications, the oscillatorbeing made of ferromagnetic material or being made of a ferromagneticmultilayer stack on top of a piezoelectric layer, wherein a surfaceacoustic wave is generated to influence said ferromagnetic material,said influencing comprising changing a ferromagnetic resonanceproperties.

In the present invention, the ferromagnetic resonance frequency may belarger than 0.5 GHz, preferably larger than 1 GHz. The angles discussedfor the different embodiments of the present invention are absoluteangles. Furthermore, where a dynamical measurement of themagneto-resistance is discussed, this means that there is a continuousmeasurement during time, or a measurement at regular times, of themagneto-resistance. Although in the present application the surfaceacoustic means are discussed, a pulsed laser also could be used as thethermal expansion of the material than could be used for induction ofstress or strain in the transport layer leading to generation of a SAWcausing effective magnetic fields due to magneto-striction. Although inthe embodiment of the present invention, one and two dimensionalsensors, cameras and devices are described, the invention also can beused to make a volumetric sensor, e.g. by placing different devices,having a two dimensional structure, on top of each other. These devicesmay use a common surface acoustic wave generating means.

Particular and preferred aspects of the invention are set out in theaccompanying independent and dependent claims. Features from thedependent claims may be combined with features of the independent claimsand with features of other dependent claims as appropriate and notmerely as explicitly set out in the claims.

It is an advantage of embodiments of the present invention that noexternal magnetisation input such as an externally induced magneticfield or an incident magnetic wave is needed for operating the device.

It is an advantage of embodiments of the present invention that thesystems allows to influence the magnetisation of the magnetic elements.Features described in specific embodiments are not limited to thesespecific embodiments and may be mutatis mutandis be applied, used orintroduced in other embodiments of the present invention. Similarly,features of dependent claims depending on a specific independent claimmay be combined with other independent claims. Although there has beenconstant improvement, change and evolution of methods and devices inthis field, the present concepts are believed to represent substantialnew and novel improvements, including departures from prior practices,resulting in the provision of more efficient, stable and reliablemethods and devices of this nature.

The teachings of the present invention permit the design of improvedmethods and apparatus for ultra fast assessment of magnetic elementswith high controllability.

These and other characteristics, features and advantages of the presentinvention will become apparent from the following detailed description,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention. This description isgiven for the sake of example only, without limiting the scope of theinvention. The reference figures quoted below refer to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 to FIG. 4 show four different practical embodiments of a devicefor ultra fast assessment of magnetic elements, wherein the magneticelement is not in direct contact with the SAW generating means,according to the first embodiment of the present invention.

FIG. 5 to FIG. 8 show four different practical embodiments of a devicefor ultra fast assessment of magnetic elements, wherein the magneticelement is in direct contact with the SAW generating means, according tothe second embodiment of the present invention.

FIG. 9 a to FIG. 9 d shows simulation results for a device whereinstrain is generated in the SAW generating means, according to the secondembodiment of the present invention.

FIG. 10 shows the resistance change in a SAW generating means being amagnetic IDT as a function of the electric field applied to it, as usedin the second embodiment of the present invention.

FIG. 11 a is a schematic representation of a shear SAW generating means.

FIG. 11 b is a schematic representation of the stress and/or strain waveinduced in the magnetic element and the corresponding effective magneticfield components created according to the present invention.

FIG. 11 c is a schematic representation of the change of magnetisationinduced by the shear SAW in a device according to the present invention.

FIG. 12 is an illustration of the effective magnetic field induced by aSAW and the magnetisation of a magnetic element in a device according tothe present invention.

FIG. 13 a is a schematic representation of a device according to thepresent invention wherein a Rayleigh SAW is induced.

FIG. 13 b is a schematic representation of the induced stress and/orstrain and the effective magnetic field components in a device accordingto the present invention.

FIG. 13 c is a schematic representation of the change of magnetisationinduced by a Rayleigh SAW in a device according to the presentinvention.

FIG. 14 a illustrates an in-plane magnetic sensor also for use atfrequencies higher than 1 GHz, according to the present invention.

FIG. 14 b is a graph representing the evolution of themagneto-resistance as a function of stress, for a magnetic element in adevice according to the present invention

FIG. 14 c is a schematic representation of the influence of the amountof anisotropy in a magnetic element in a device according to the presentinvention.

FIG. 15 a is a schematic representation of a density of magneticcomponents between a SAW generating means according to an embodiment ofthe present invention.

FIG. 15 b is a schematic representation of a magnetic component, i.e.for example a spin valve, according to the present invention.

FIG. 15 c is a graph of the magneto-resistance as a function of stressfor a spin valve magnetic component according to the present invention.

FIG. 15 d is a schematic representation of the response of a magneticelement in the “1” state, i.e. for parallel magnetisation of the layers,(central graph) and the response of a magnetic element in the “0” state,i.e. for anti-parallel magnetisation of the layers, (right graph) for adistinct surface acoustic wave (left graph).

FIG. 16 gives a schematic representation of a switching scheme formagnetic layers and components through SAW activation, according to thepresent invention.

FIG. 17 gives a schematic representation of a device wherein bothRayleigh SAWs and shear SAWs can be applied, according to the presentinvention.

FIG. 18 a gives a schematic representation of the magnetisation fieldsoccurring in a magnetic element upon SAW activation, according to thepresent invention.

FIG. 18 b gives a schematic representation of a radio frequentmechanical resonator, using an induced stray field according to thepresent invention.

FIG. 19 gives a schematic representation of the application of SAWinduced magnetisation as driving force for use in magnetic logic.

FIG. 20 is a graph of the frequency range that can be used for the SAW.

FIGS. 21 to 24 show experimental results of embodiments of the presentinvention.

FIGS. 25 and 26 illustrate that in embodiments according to the presentinvention the damping towards the equilibrium state, represented by theGilbert damping parameter α, can be actively controlled by the amplitudeand the phase of the SAW.

In the different figures, the same reference figures refer to the sameor analogous elements.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. Where the term “comprising” is used in thepresent description and claims, it does not exclude other elements orsteps.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in thedescription and the claims are used for descriptive purposes and notnecessarily for describing relative positions. It is to be understoodthat the terms so used are interchangeable under appropriatecircumstances and that the embodiments of the invention described hereinare capable of operation in other orientations than described orillustrated herein.

For the purpose of the present invention the expression ‘A in contactwith B’ refers to spatial and/or structural configurations of A and Bfor which the propagation of surface acoustic waves and/or strain wavesbetween A and B is possible.

In a first embodiment a device 100 for switching and/or determiningand/or manipulating and/or changing a magnetisation state of aferromagnetic component is described. The device 100 comprises a surfaceacoustic wave (SAW) generating means 102, a transport layer 104 to allowpropagation of the generated SAW and in which a SAW has the form of astress and/or strain wave, and a magnetic element 106, also calledmagnetic cell or cell, whose state can be switched or determined orassessed using the stress and/or strain wave. Strictly spoken, thetransport layer 104 is also part of the SAW generating means 102.Surface acoustic waves are generated by the SAW generating means 102 andneed a material for their transfer, which is the transport layer 104.Therefore, the SAW generating means 102 and the transport layer 104belong together. However, for the ease of explanation, in the furtherdescription, the SAW generating means 102 and the transport layer 1041will be discussed separately. According to the present invention, themagnetic element typically consists of ferromagnetic material, having atypical ferromagnetic resonance frequency. Although the device 100 canbe adjusted to work in other frequency ranges, the RF frequenciestypical for FMR materials are interesting as they allow high speedswitching, sensing, etc. The SAW generating means 102 can be e.g. aninter digitated transducer (IDT) which is deposited on and hence is inphysical contact with a transport layer 104. However, the invention isnot limited thereto. The SAW generating means 102 may furthermore be forexample a laser, for instance deposited on a transport layer 104,whereby SAW's are generated by laser pulses.

IDT's are known from their use as RF-filters. They have reached a stagebeyond development, and are widely used products. They are, even in theseveral GHz region, a commercially available product, known by theperson skilled in the art. Although in principle all types of surfaceacoustic waves can be used, the SAW choice can be mainly reduced to thedistinction between Rayleigh and shear waves, i.e. SAWs which areperpendicular to the surface plane and SAWs which are in-plane. Thetransport layer 104 may typically comprise a piezoelectric material orpreferably is made of piezoelectric material, but can also be any othermaterial in which the propagation of a SAW is possible and wherein theSAW wave has the character of a strain wave. The magnetic element 106 tobe addressed is positioned on top and in physical contact with thetransport layer 104. In the present embodiment, the magnetic element 106is not in contact with the SAW generating means 102, i.e. it is isolatedtherefrom. The magnetic element 106 may be part of a magnetic component.A broad range of magnetic components may be used with embodiments of thepresent invention such as e.g. tunnel junctions, spin valves, singlelayers, . . . . A typical example of a magnetic component that can beused is an MRAM device. The choice of the magnetic element 106 andpossibly the magnetic component used in the device 100 according to thepresent invention, depends on the application.

In order to use the device 100 described above, i.e. to switch or assessor determine the state of or manipulate the state of the magneticelement, the following method is used. The SAW generating means 102generates a SAW with a certain frequency ν_(SAW). This SAW ispropagating further into the transport layer 104, which is in physicalcontact with the SAW generating means 102. The SAW generates a timedependent strain at every spot of the transport layer 104. Since themagnetic element 106 or cell is placed in physical contact with thetransport layer 104, the strain wave, which has been induced by the SAW,will propagate also into the magnetic element 106. The strain wavegenerates an associated effective magnetic field in the magnetic element106, which can interact with the magnetisation state of the cell.Interacting with a magnetisation state of a magnetic element 106 can befor instance mean assessing, determining, manipulating or changing themagnetisation state. It has been found that magnetic elements 106 havethe capability of efficiently absorbing SAWs, i.e. are influencedefficiently by the corresponding strain wave, when their frequency isclose enough to the FMR frequency of the magnetic element 106. Thisenergy-loss will be converted in the magnetic element 106 which can bepart of a layer in a change in magnetic state. In other words, the SAW,generated by the SAW generating means 102, generates the necessarystrain for the magnetic element(s) 106 to change its magnetisationstate.

So, RF-SAW devices can be used to apply the necessary stimulus tooperate magnetic layers at their FMR frequencies, and altering theirproperties, which may be especially the magnetisation direction but alsomay be other properties as e.g. switching behaviour, coercivity,biasing, permeability, susceptibility, damping or absorption behaviour.The latter can be made visible by the width of the absorption peak.These properties all are related to the magnetisation, i.e.magnetisation state of the magnetic element. This altering of propertiesallows to switch or assess or determine the magnetisation state of amagnetic element at very high frequencies, i.e. typically above 1 GHz.Thus instead of using external magnetic fields, the present inventiondiscloses a novel method which uses magneto-elastic energy conversion tochange the magnetisation state of the ferromagnetic element 106, or inother words, this solution uses magneto-elastic energy conversionbetween a magnetic element 106 and a SAW in a piezoelectric layer tointeract with the magnetisation state of the magnetic element 106.

Different configurations can be provided for the device 100 according tothe present invention. These configurations can be determined by thechoice of SAW type, i.e. Rayleigh and/or shear SAWs, the way thepiezoelectric material is provided and the magnetic element 106 chosen.Some preferential configurations combining specific choices are shown inFIGS. 1 to 4. Nevertheless, the present invention is not limited by theconfigurations shown, but only by the claims.

In FIG. 1 to FIG. 3 a transport layer 104, being a piezo-electric film,is deposited on a substrate 108. This substrate 108 may be any substratepossible, such as glass or any type of plastic, or a semiconductorsubstrate, such as silicon or germanium, GaAs, . . . . Thepiezo-electric film may be made of any piezo-electric material in whichthe creation of a SAW, corresponding to a stress wave with associatedcreation of stress and/or strain in the film, is possible, such as e.g.GaN or quartz. Furthermore in FIG. 1, the SAW generating means 102 andthe magnetic element 106 both cover different parts of thepiezo-electric film, such that the SAW generating means 102 and themagnetic element 106 are at least partly in the same plane. In principlea single SAW generating means 102 is sufficient to generate thenecessary stress in the piezo-electric film and consequently in themagnetic element 106 to influence its magnetisation state. In thespecific configurations described, at least two SAW generating means 102are provided, which can be positioned at each side of the magneticelement 106, preferably, but not necessary, symmetrically.

In FIG. 2, the magnetic element 106 is patterned and buried by thepiezo-electric film or transport layer 104, i.e. between thepiezo-electric film or transport layer 104 and the substrate 108, whilethe SAW generating means 102 still are on top of the piezo-electric filmor transport layer 104.

In FIG. 3 a non-patterned magnetic element 106, i.e. a full layer, isshown extending between the substrate 108 and the piezo-electric film ortransport layer 104, whereby the SAW generating means 102 still are ontop of the piezo-electric film or transport layer 104.

Another configuration is shown in FIG. 4 whereby the piezo-electricmaterial is provided as a substrate 110. This substrate 110 may consistof any piezo-electric material in which the creation of a SAW,corresponding to a stress wave with associated creation of stress and/orstrain in the film, is possible, such as e.g. GaN or quartz. The SAWgenerating means 102 and the magnetic element 106 can then be positionedor deposited on top of the piezo-electric substrate 110 in a patternedway. It may be advantageous that there is no direct physical contactbetween the SAW generating means 102 and the magnetic element 106, asdepicted in the configurations of FIG. 1 to FIG. 4. The latter may e.g.allow to avoid shortcutting the IDTs. Since the electric field used togenerate a SAW has to be applied between distinct transducers, noshortcuts between the fingers are allowed. Furthermore, in certainembodiments it is advantageous that the magnetic elements are not underthe same electric field as the IDTs. This makes their individualassessment more straightforward.

In another viewpoint this can be seen as a need for the SAW andassociated stress wave to have a transport means or layer to reach themagnetic element, since a SAW generating means is mostly not able togenerate a SAW directly in such a magnetic element, as is for instancethe case for an IDT.

In a second embodiment, the invention relates to a device 100 forswitching and/or determining and/or manipulating and/or changing amagnetisation state of a ferromagnetic component, comprising the samecomponents and functionalities as described in the first embodiment, butwherein the magnetic element 106 is in direct contact with the SAWgenerating means 102, such as e.g. the IDT or wherein the IDT is itselfthe magnetic element 106 of interest. The magnetic element 106 in directcontact with the SAW generating means 102 may e.g. be deposited directlyon top of the SAW generating means 102. Such configurations are depictedin FIG. 5 to FIG. 8. FIG. 5 indicates the configuration wherein amagnetic element 106 is deposited on top of a SAW generating means 102whereas FIG. 6 indicates the configuration wherein the magnetic element106 of interest is the SAW generating means 102 itself. In bothconfigurations a transport layer 104 and a substrate 108 is present.FIG. 7 and FIG. 8 indicate similar configurations wherein the substrate110 is a piezo-electric material, thus avoiding the need for anadditional transport layer 104. In these configurations, the SAWgenerating means 102 generates a SAW and associated stress wave in thetransport layer, which may be part of the SAW generating means 102 orpart of the substrate 110. This stress wave is felt by the SAWgenerating means 102, an effect which has been experimentally proven foran IDT. The SAW generating means 102 deforms under such stress exertionand generates a stress in the magnetic element in direct contact withthe SAW generating means 102. In case the magnetic layer of interest isthe SAW generating means 102, e.g. IDT, itself, the stress is directlysensed. Experimental data supported by simulations indicate that strainis generated in the saw generating means 102, e.g. IDTs. Thecorresponding simulation results are shown in FIG. 9. FIG. 9 a shows aconfiguration having 2 sets of 2 fingers and the potential they feel ata certain moment, determined by the maximum amplitude of the SAWgenerating electric field of the SAW. FIG. 9 b and FIG. 9 c representthe strain in the length, respectively the width direction of thefingers for the potential as depicted in FIG. 9 a. FIG. 9 d shows thestrain in the width direction for the inverse potential. These figuresshow that the fingers experience an alternating strain, corresponding tothe stress a SAW generates in a layer on top of it. Depositing amagnetic element directly on top of an IDT couples the strain generatedin the IDT to the magnetic element, thus giving the necessary stimulus.FIG. 10 shows the resistance change in a magnetic IDT in function of theelectric field applied to it. The strain effectively couples in themagnet to the magnetic properties (AMR effect), demonstrating thepossibility to use the IDT as magnetic element of interest.

The present invention offers different advantages. There is no need forthe use of large currents to create a magnetisation switch, nor is theira need for using reference cells. Furthermore, it provides lessstringent timing issues and allows for simultaneous read and writepossibilities. No external fields are needed. Moreover, in mostapplications, no exact read-out value is necessary, just the phase.

In a third embodiment, a device 100 according to, i.e. having the samecomponents as, any of the configurations described in the previousembodiments is provided, whereby the SAW generating means 102 can applya shear SAW. A shear SAW, as depicted in FIG. 11 a, is a surfaceacoustic wave that is launched by a SAW generating means 102, e.g. anIDT, and that makes the surface deform in a sinusoidal manner in plane(X-Y plane), generating a shear strain on every spot where the SAWpasses. The magnitude of the shear strain, at given time and place, isdepending on several parameters, from which the most important are thevoltage applied on the SAW generating means 102 and the phase of thewave. So, a layer deposited in the path of the SAW, at its fixedlocation, senses a shear strain which changes in magnitude over time.

If this layer is a ferromagnetic layer, then the shear strain generatesan effective magnetic field in the layer, as shown in FIG. 11 b. As themagnetic elements 106 are placed on top and in physical contact with thetransport layer 104 which experiences a strain wave or in direct contactwith the SAW generating means 102 which experience a strain wave, themagnetic element 106 in the different embodiments of the presentinvention will experience the strain wave, and the induced associatedeffective magnetic field. More in detail, the shear strain can bedecomposed in compressive and tensile strain components, as shown inFIG. 11 b. These components can be represented by a component parallelwith the easy axis of the magnetic element 106 and a componenttransversal on this component. The easy axis defines the preferredmagnetisation direction of the magnetic material. Consequently, a straincomponent parallel and a strain component perpendicular to this axis canbe distinguished. This strain will induce an associated magnetic fieldin the magnetic element 106.

Depending on the sign of the magneto-striction of the magnetic material106, which is a typical material characteristic, the strain induced bythe SAW generates a field in the x-direction if the material has anegative magneto-striction coefficient or a field in the y-direction ifthe material has a positive magneto-striction coefficient. Consideringthat the magnitude of the strain is time-dependent, also the magnitudeof the effective field changes in time. This causes a change inmagnetisation and overall magnetic properties. If this SAW activation isperformed at or near the FMR frequency of the magnetic material 106,then absorption of the SAW will be higher, and the magnetisation will beable to respond in similar way as shown in FIG. 11 c. It is to be notedthat no assumptions are made on magneto-striction nor on easy axisdirection as compared to the wave direction. In the left drawing of FIG.11 c the SAW begins to act on the magnetisation. Since the beginning ofthe SAW has a small amplitude, the magnetisation will be drawn towardsthe direction of the effective field. As the strain gets higher, therotation of the magnetisation gets faster and has an overshoot, as shownin the central drawing of FIG. 11 c. Then, by lowering the strain untilzero, the magnetisation will return and eventually start precessingaround the effective field, as shown in the right drawing of FIG. 11 c.

In the description of the effect of a shear strain wave on themagnetisation, the magnetic element 106 has been considered asinfinitesimally small, which is an ideal case. In practice a similarbehaviour can be obtained by making the elements 106 as small aspossible with respect to the wavelength of the SAW λ_(SAW). Choosingsuch small sizes implies that the strain is comparably large oressentially the same at all positions within 1 cell. Bigger sizes ofcells will have a strain and thus an induced effective magnetic fieldwhich will differ substantially between two points in a cell. In otherwords these larger sizes will have inhomogeneous magnetisationdistributions, resulting in magnetisations that will be rotating inperiods, as dictated by the SAW wavelength λ_(SAW). Problems should notarise for element sizes smaller than ¼ the wavelength of the SAW.

An important parameter of this activation is the angle θ between theeasy axis of the magnetic layer 106, and the direction of the effectivefield, as shown in FIG. 12. Maximum response is obtained for the anglethat maximises the momentum, i.e. θ=90°. For θ=0°, no effect will beobserved as the magnetisation is already in the direction of thegenerated effective magnetic field. For a material with negativemagneto-striction this implies that the magnetic material should be putwith the easy axis perpendicular to the SAW direction. A positivemagneto-strictive material requires an easy axis parallel to the SAWdirection. In general; for negative magneto-striction material, the easyaxis should be not parallel to the SAW direction, for positivemagneto-striction material the easy axis should be not perpendicular tothe SAW direction.

If e.g. a magnetic material with a negative magneto-striction is used, atensile stress causes an effective magnetic field that is perpendicularto the direction of the stress, while a compressive stress causes aneffective magnetic field parallel to the direction of the stress. InFIG. 11 b it is illustrated that strain wave in the x direction createscompressive stress along the x direction, while the tensile stressoccurs along the y direction. This implies that the magnetic field,created by both stress components is oriented along the x-direction. Theeffect of the effective magnetic field on the change in magnetisationorientation depends on the angle between the direction of the effectivemagnetic field and the direction of the magnetisation. In this case,maximum effect will be obtained if the easy axis is along the ydirection, i.e. perpendicular to the x-direction, which is the directionof the effective magnetic field. The magnetisation direction thus shouldbe perpendicular to the strain wave direction. For positivemagneto-striction materials, the magnetisation direction should beparallel to the strain wave, based on a similar deduction.

Thus the application of a shear SAW allows to directly access the FMRfrequency of a magnetic layer/component 106, thus altering itsproperties, i.e. switching behaviour, coercivity, biasing,susceptibility, absorption behaviour leading to e.g. damping . . . , atRF frequencies, i.e. frequencies higher than 1 GHz.

In a fourth embodiment, a device 100 according to, i.e. having the samecomponents as, any of the configurations described in the first orsecond embodiment is provided, whereby the SAW generating means 102 areadjusted to apply a Rayleigh SAW. A Rayleigh SAW, as depicted in FIG. 13a, is a surface acoustic wave that is launched by a SAW generating means102, e.g. an IDT, and that makes the surface deform in a sinusoidalmanner perpendicular to the plane, generating a strain on every spotwhere the SAW passes. The magnitude of the Rayleigh strain, at giventime and place, is depending on several parameters, from which the mostimportant are the voltage applied on the SAW generating means 102 andthe phase of the wave. So a layer deposited in the path of the SAW, atits fixed location, senses an in magnitude, as well as in sign, changingstrain. If this layer would be a ferromagnetic layer 106, then thestrain would generate an effective magnetic field in the layer, as shownin FIG. 13 b. For a Rayleigh wave passing through a ferromagneticmaterial at a certain position, the wave first causes a tensile stressin the magnet (first half period), followed by a compressive stress inthe second half period. For a negative magneto-strictive material thiscauses, first, an effective field perpendicular to the SAW direction(y-direction), followed by a field in the direction parallel to the SAW(x-direction). For a material with positive magneto-striction thedirections are opposite. If this SAW activation is performed at FMRfrequency of the magnetic material, then absorption of the SAW will behigher, and the magnetisation will be able to respond in a way as shownin FIG. 13 c. The situation shown is the case for negativemagneto-strictive material and the first period of a SAW, or for apositive magneto-striction material and the SAW shifted 180 degrees. Inthe left drawing of FIG. 13 c the SAW acts, by means of its inducedeffective magnetic field, on the magnetisation in the y-direction. Thiscauses the magnetisation to rotate around the effective field and hencecoherently switch the magnetisation. When the material is switched, theSAW applies a field in the direction of the easy axis (x), and themagnetisation relaxes around its switched equilibrium, as shown in theright drawing of FIG. 13 c.

An advantage of switching like this is the less stringent requirementson timing, compared to magnetic field-induced switching by currentcarrying conductors. If for example the frequency of the SAW is notclose enough to the FMR frequency, then the magnetisation won't beexactly switched, but has passed the switching threshold, when thesecond period begins, or the magnetisation will have overshot theequilibrium position by an amount that wouldn't cause the magnetisationto return to its initial value. Here the field in the x-direction playsan important role in returning the magnetisation to the switchedposition. This effective field pulls the magnetisation in thex-direction, hence stabilising the switching and lowering the ringingthrough an improved damping mechanism. By ringing is meant that themagnetisation rotates around the direction of an equilibrium state,which typically will be the direction of the easy axis. The damping ofthe magnetisation, i.e. the relaxation to its equilibrium state, isaccelerated by applying an effective magnetic field in the direction ofthe equilibrium state. This description of the effect of a Rayleighstrain wave on the magnetisation is made for an element that isinfinitesimally small. In practice a similar behaviour can be obtainedby making the elements as small as possible with respect to λ_(SAW).Choosing sizes like this implies that the SAW or strain wave iscomparably large at all positions. Larger sizes will have inhomogeneousmagnetisation distributions, the magnetisations will be rotating inperiods, as dictated by the SAW period. Problems should not arise forelement sizes smaller than ¼ the period of the SAW.

An important parameter of this activation is the angle between the easyaxis of the magnetic layer 106, and the phase of the strain wave. For amaterial 106 with negative magneto-striction this implies using thefirst period of a sine and putting the easy axis parallel to it. Apositive magneto-strictive material requires an easy axis perpendicularto the SAW when using the first period. This is obtained based on thesame principles as discussed for the shear wave. This is due to the factthat, for e.g. a negative magneto-strictive material, in order to obtainan optimum magnetic moment, the easy axis has to be perpendicular to theeffective magnetic field. As during the first half period of the wave,the effective magnetic field is oriented perpendicular to the directionof the SAW wave, the latter being the x-direction, the easy axis thushas to be in parallel with the SAW direction. This can be seen in FIG.12. For a positive magneto-strictive material, the effective magneticfield during the first half period is oriented in the x-direction andthus the easy axis should be oriented in the y direction.

Thus, the application of a Rayleigh SAW allows to directly access theFMR frequency of a magnetic layer/component 106, thus altering itsproperties, i.e. switching behaviour, coercivity, biasing,susceptibility, permeability, . . . , at RF frequencies.

It will be obvious to the person skilled in the art that otherconfigurations are possible, e.g. by changing the position of the SAWgenerating means 102, e.g. placing them in the y direction instead ofthe x direction. The positioning of the magnetic elements 106, and morespecifically their easy axis, then also changes mutatis mutandis.

In a further embodiment, an in-plane magnetic sensor 200 is describedworking also at frequencies higher than 1 GHz. The magnetic sensor 200can be used for detection of several different parameters. Here theapplication of stress sensors as well as field sensors will be treatedas the same type, because, by magneto elastic interaction, they aregenerally interchangeable. The main difference is the lack of sense forstrain. A magnetisation points in one specific direction, but stressworks in both senses, for instance + and −x-direction.

The effect of a SAW on a magnetic sensor 200, which as example is a spinvalve, but may also be for example a tunnel junction or AMR-sensor, . .. is to constantly access its magnetisation. The sensor 200 is e.g. aspin-valve and may comprise a sensing layer 202, a barrier layer 204, asecond layer 206 and a biasing layer 208. The perturbation given to asingle sensor element 200 is that the free layer rotates itsmagnetisation between two states, determined by the angle of theeffective field (stress related) and its magnitude. This is shown inFIG. 14 a. This translates to a change in resistance through the MagnetoResistance-effect (MR-effect) as shown in FIG. 14 b. By monitoring ofthe change in resistance, information about the sensor is obtained. So,when the sensor 200 is placed in a changing environment, its propertieschange, including its response to the activating SAW. In FIG. 14 c, thechange response is shown. As can be seen, two main properties change:the slope of the response, and therewith the magnitude, and the initialpoint. The slope of the response is depending on the amount ofanisotropy in the equilibrium direction. This means that when acomponent has more anisotropy in its equilibrium state, it is harder toactivate it with the SAW. This corresponds to a decreasing slope, hencecausing a lower response of the sensor 200 on the SAW. The second changeis related to the equilibrium direction. If this is no longer alignedwith the easy axis of the sensor 200, the sensor 200 doesn't return toits parallel sensing layer-fixed layer position. These two parametersare a measure for the change of environment.

In other words, a magnetic component, spin-valve or tunnel-junction isconstantly given small angle perturbations to its magnetic state. Allchanges in anisotropy, caused by e.g. external fields, stress, . . . ,will change the response of the components. By looking at this responsea conclusion can be drawn concerning the amount of anisotropy in thesystem, as well as its direction. These two properties give a measurefor the changes in environment. This principle can be used in ultra fastmagnetic field sensors, stress sensors, etc.

The previous explanation implies that the size of the sensor 200 issmall enough to have the magnetisation respond in a spatially uniformway to the SAW excitation. Preferably sizes of up to a quarter of thewavelength of the SAW are used. When a larger sensor is required, apatterned and series connected sensor can be used. Such a sensor 300 isshown in FIG. 15 a. This avoids that the spin wave generated in thelarge sensor creates an MR-response that is averaged to zero, whichcould be the case if a single-entity large component is used.Alternative embodiments, whereby the sensor is in direct contact withthe SAW generating means, in agreement with the second embodiment, alsocan be used.

An advantage of using a patterned component according to the presentinvention is that a magnetic camera with micron resolution can beconstructed. By separately reading out the composing sensors, a 2-Dimage of the magnetic environment can be generated. Since the read-outis done at frequencies higher than 1 GHz, 10 ⁹ elements can be read-outper second, one at a time. This makes it a fast, high-resolution sensor.

A problem that can arise both in a bulk sensor, which can comprise aseries of sensors, and in a camera is that the frequency of FMR shiftswith altering environmental properties. To avoid this, a feedback schemecan be provided to solve this problem. Moreover, the frequency shift canbe a measure for the magnitude and direction of a global effective fieldpresent in the camera or bulk sensor. By adding this to the small-fieldresponse measured by the separate sensors, a large range sensoroperation can be assured.

The present invention also relates to a read-out scheme for exchangedbiased spin-valve or tunnel-junction structures. The exchange biasedspin-valve or tunnel-junction read-out scheme is directly based on theeffect of a surface acoustic wave on magnetic materials. A possibleconfiguration is shown in the device 300 of FIG. 15 a. Here severalcomponents 200 are deposited between the SAW generating means 102 suchas e.g. IDTs, generating the SAW. The density of these components 200depends on considerations made when describing the effect of SAWs onferromagnetic materials. Hence, the elements can be easily placed at apitch of a quarter of the wavelength of the SAW. This is in its turndictated by the FMR frequency of the magnetic element. A typical valuefor some typical materials is approximately 2 micron. This allows toobtain a density of 25×10⁶ elements/cm².

The effect of a SAW on a magnetic component 200 is depicted in FIG. 15b. In this figure the different composing layers for the components areshown. The materials used for different magnetic elements differ, i.e.for example for tunnel-junctions the barrier is most common to be anoxide, whereas for a spin-valve this is a non-magnetic metal. A SAWcauses a perturbation of the magnetisation, which is different fordifferent ferromagnetic layers. This depends on the properties of themagnetic material. Choosing a stress-sensitive material, i.e. a materialhaving a high magneto-striction to amount of anisotropy ratioλ_(S)/K_(u), as sensing layer 202 and a stress-insensitive material,i.e. a material having a low magneto-striction to amount of anisotropyratio λ_(S)/K_(u) as second layer 206, allows that the magnetisation isonly affected in one layer. An example of a corresponding component isshown in FIG. 15 b, wherein the bottom layer of the structure is asensing layer 202 and the second layer 206 is a stress-insensitivelayer. These material properties are also valid for the sensorembodiment according to the present invention. A barrier layer 204 alsois provided. The anti-ferromagnetic layer is included in the stack tobias the component 208. This layer should also be stress-insensitive.The activation can be done by both shear as Rayleigh waves. Care shouldbe taken not to switch the component, hence destroying its state. Theway the perturbation is perceived is shown in FIG. 15 c. In the figure,a magneto resistance measurement is shown. This depicts the change inresistance in function of the applied stress. This stress can begenerated by a SAW. There are two possible states for the component tobe in. The bottom rising line belongs to the “1” state, i.e. forparallel magnetisation of the layers, the top descending line belongs tothe “0” state, i.e. an anti-parallel configuration of the layers. If aSAW, having a form as shown in the left figure of FIG. 15 c is passedthrough the component, there are two possibilities. If the spin-valve isin the “1” state, the effective field causes a resistance change asshown by the dark grey markers. This gives a resistance change as shownin middle picture of FIG. 15 c. A “0” gives a resistance change that isin anti-phase with the activating signal, as shown in the right pictureof FIG. 15 c. By comparing phases of the input and output signal, aread-out can be made, without the need for an absolute resistancemeasurement, nor the need to compare the output with a reference cell.The phase of the input signal can be easily derived by calculation.

The present invention furthermore relates to a scheme or method forswitching of magnetic components. This follows directly from the effectof the SAW on the magnetic material. The method is based on any of thedevices presented in the first, second or third embodiments. Smalldifferences occur depending on the choice between a shear wave or aRayleigh wave.

If a shear wave is used, the wave can give an impulse to themagnetisation, the momentum being maximum when the angle between themagnetisation direction and the effective field is 90 degrees. Whenapplying the SAW, a perturbation to the magnetisation will be given. Themagnitude hereof is depending on the applied voltage in the SAW creatingmeans. Above a certain threshold voltage, i.e. above a certain magnitudeof the SAW, the magnetisation precession will rotate far enough to havethe magnetisation closer to the anti-parallel magnetisation direction.This implies that, when the activating SAW is turned off, themagnetisation will relax towards a switched equilibrium. With thisswitching method, timing becomes an important factor. Switching the SAWon and off (or increasing and decreasing its magnitude) should be doneat the exact right time, to avoid not reaching the switching thresholdor overshooting the switched position.

If a Rayleigh wave is used, an impulse is also given. The impulse hastwo perpendicular effective fields as response. Switching occurs byapplying a large enough magnitude of SAW, hence in the first half periodprecessing the magnetisation to the switched position, and consequentlypulling the magnetisation towards the switched position. Timing is inthis case less of an issue, since the entire second half period of theSAW satisfies the switching criteria. This implies that for switching,the Rayleigh waves configurations are the most effective and easy toimplement.

The above description referred to single elements. When an array ofmagnetic components 200 needs to be addressed, an additional parameterhas to be included, which is element specific. Therefore, the read-outcontacts 210 can be used, as shown in FIG. 16. Sending a current throughthe element, e.g. a spin valve as shown in FIG. 16, generates a magneticfield, that can help switching by lowering the anisotropy in the easyaxis direction. So, to avoid switching all the elements simultaneously,a SAW magnitude is chosen that lies at a safe distance, i.e. to avoidunintended switching, below its threshold value. The field generated bythe current lowers this threshold below the SAW amplitude, so switchingthe selected element. Except for the element specificity of thisapproach, timing issues can be approached differently. It is not the SAWwhich has to be timed (what can be difficult because of the nature ofSAWs, and its response to single pulses), but the selecting current isto be timed. This is especially an improvement for the shear waves.Another advantage that comes from this scheme is the fact that severaldifferent operations can be combined. The SAW used causes perturbationsrequired for the read operation, as well as for writing. Only themagnitude of the read/write current is determining whether reading orwriting is selected. Moreover, different write operations can be donetogether. Thus a switching scheme for magnetic layers and componentsillustrates activation through SAW. At FMR frequency the magneticlayer/component can be given a large enough excitation to switch itsmagnetisation. Furthermore, for a Rayleigh wave, since it generatesperpendicular effective fields, overshoot of coherent switching can beavoided. Bit-selectivity in an MRAM-like scheme is obtained by using anot-switching SAW, and by locally applying a small field. This can beachieved by sending a current through the structure of interest.Reading-out multiple elements in the same time slot requires someprocessing logics.

In a further embodiment 400, use is made of both Rayleigh SAWs and shearSAWs on the same surface. This is made possible by a design in which theSAW generating means 102, 402, which may for example be IDTs, for theRayleigh SAWs and the shear SAWs are at an angle of substantially 90°.This is shown in FIG. 17. In such a design a Rayleigh wave is running at90° with respect to the shear wave. The shear wave generates a smallerperturbation, which makes it suitable for read-out activation. Writingthen can be done by the Rayleigh wave. Bit-selectivity is obtained bysending a current through the element in order to generate a magneticfield. The current can be replaced by any other means that changes theeffective field of one magnetic element and thus lowers the switchingthreshold value., i.e. ways such as magnetic fields generated by currentconductors close by, or thermally or spin torque (angular momentum)induced magnetic changes generated by heaters, laser pulses, or currentsthrough the element. This design allows a simultaneous read and writeoperation. Different write operations can be done together. Reading-outmultiple elements in the same time slot requires some processing logic.As the velocity of the shear wave is higher than for Rayleigh waves forthe same design of the IDTs, it is possible to have a shear wave runningat an angle of 90° with the Rayleigh wave and at a slightly higherfrequency (other position on absorption peak). Thus the shear frequencyis the read-out frequency, having a smaller magnetic perturbation, andthe Rayleigh frequency can be the writing frequency.

In another embodiment, a RF mechanical resonator 500 is provided. The RFmechanical resonator 500 is based on the devices for switching andassessing a magnetisation state of a ferromagnetic component, asdescribed in the above embodiments. The effect of a SAW on a magneticlayer 502 is stated to cause a perturbation of the magnetisation. For aRayleigh as well as a shear wave this magnetisation change also has anout-of-plane component. Since precession tends to happen in the planeperpendicular to the effective field, a large out-of-plane component isgenerated, which will be almost completely countered by the largedemagnetisation fields generated by the thin film magnetic layer 502.This interaction of fields and forces make a net stray field to arisefrom the magnetic layer 502, as shown in FIG. 18 a. Here a Rayleigh wavecauses a magnetisation rotation as depicted in the FIG. 18 a, causing analternating stray field. For a shear wave, the same reasoning can beused.

The stray field can then be used for activation of a resonator. In FIG.18 b, a tip, e.g. an AFM-like tip 504, is brought close to the magneticlayer 502. To have the stray fields interact with the tip 504, thelatter was chosen to be magnetic. This is a GHz-compatible tip 504,which typically is coated with a magnetic material, Hence, the tip 504feels the magnetic field, and through the periodicity of this field, thetip 504, which is mounted on a cantilever-type structure 506 getsattracted and repelled by the SAW-activated magnetic layer. If the tip504 chosen is designed to have its resonance in the right frequencyrange, i.e. near the FMR frequency of the magnetic layer 502, the tip504 can be brought into resonance. This mechanical resonance generatedby the tip 504 can then be the output of the system. In other words, themagnetic force exerted by the magnetic field will be sensed by the tip504 and converted to a mechanical resonance.

In a further embodiment of the invention, the use of a SAW as drivingforce in magnetic logic is described. The use of magnetic logic is knowne.g. from Cowburn et al. in Science 287, (2000) p 1466. Magnetic logicelements are the magnetic equivalent of electronic components. Inmagnetic logic elements, the state of the element is not defined by thecharge but by the magnetisation of the element. This requires theconstant presence of driving magnetic fields. Due to severalconsiderations, among which power consumption plays a crucial role, theuse of external fields is not a preferred situation. In the embodimentaccording to the present invention, the magnetic field required forswitching is created by an effective magnetic field induced by SAWs,i.e. using the devices described in the previous embodiments. In FIG. 19a small part of a possible magnetic logic circuit is shown. Propagationof magnetic information in these magnetic networks can thus be assistedby usage of SAW-generated magnetic switching instead of an externallyapplied field. This also gives the opportunity to add extra inputsthrough local magnetic fields and by the use of saw absorbing magneticlayers that can be field-tuned for passing/not-passing the saw throughto the magnetic logic array. Magnetic logic circuits work by transportof magnetic information. These logic arrays have an input bit, whichdecides whether the information can move up and down the line. Thedriving force for the movement is the magnetic field. In this way, byusing the SAW as the driving force, a logic system without externalfields is created. In this example, this is a logic AND gate between theinput bit and the “clock” (SAW). The output will only have analternating output bit when the SAW is on as shown in FIG. 19 a and theinput bit is set to allow rippling of the excitons through the lineshown in FIG. 19 a. In all other settings for the inputs, no rippling ispossible, hence meeting the properties of an AND-gate. NOT-gates alsoare known, e.g. from Cowburn et al. in Science 287, (2000) p 1466. Forthese gates an alternating field is required. The SAW is also suited forthis application. Generally speaking, the SAW-FMR principle can beapplied in all magnetic logic to replace external fields. Theapplication of SAWs also implies the operation frequencies to be in theGHz region. This is the frequency range in which currently theelectronic logic operate, making magnetic logic a valuable alternative.

In a further embodiment according to the present invention, a method foractive compensation of switching behaviour changes is provided. TheSAW-FMR devices have to be operated at a certain frequency to be at itsideal operating setting. This frequency needs to be equal to or close tothe FMR frequency, as dictated by the magnetic element. Tuning thedevice used can be performed by the choice of material, the shape of theelement, the thickness of the element, all these parameters are designparameters and should be chosen in order to determine the operationfrequency. In other words, in order to allow operation, the SAW deviceshould have a reasonable bandwidth around the FMR frequency of themagnetic element. In order to allow compensating of changes in switchingbehaviour and to allow operating in a stable regime, the SAW is to belocked just below FMR frequency. At this frequency the SAW has asemi-linear frequency-attenuation response, as shown in FIG. 20. Thelock-in frequency is indicated with a cross. By locking it atnot-maximum attenuation, a means is created to detect changes inconditions of the system by monitoring the SAW attenuation. If, forexample, the attenuation becomes higher, then there is a shift ofconditions so that the systems FMR frequency gets lower, hence thefrequency has to be lowered. If the attenuation diminishes, then the FMRfrequency is increasing, and the SAW frequency has to be increased. Theuse of this way of compensation is only based on the SAW attenuation andprovides an easy way of correcting for drift of the characteristics. Forexample, thermal effects, as well as magnetic fields from theenvironment can be accounted for. This method may be applied to thedifferent methods described in different embodiments of the presentinvention. The corresponding devices may be adapted to operate accordingto the present embodiment.

In a further embodiment of the invention, some experimental results on aRayleigh SAWs will be discussed. In this embodiment, the SAW generatingmeans is an IDT positioned onto a GaN layer. The devices work at 2.7GHz. FIG. 21 shows a transfer characteristic of the device according tothis embodiment. The IDTs have a wavelength of 4 micron. The peaks inthe insertion loss are the operating frequencies of the SAW devices. Twomain peaks are observed, i.e. at 1.46 GHz, which is the frequencycorresponding to the first harmonic, and one at 2.8 GHz, which is thesecond harmonic.

Furthermore, strain dependence of magnetic materials and of componentscomprising magnetic materials is investigated. This strain is generatedby 3-point bending (FIG. 22) as well as by piezoelectric layers (FIG.23). Rotation of magnetisation was achieved, showing the possibility tointerchanged magnetic fields and stresses. The results shown in FIGS. 22and 23 are results obtained at low frequencies. All results have beendemonstrated on single layers as well as magnetic components. FIG. 22 ashows the measurement in the easy axis direction of a tunnel junctionwith the following layer sequence:Ta/Ni₈₀Fe₂₀/IrMn/Co₉₀Fe₁₀/AlO_(x)/Co₉₀Fe₁₀/Ni/Si. The hysteresis loopsoriginating from the free layer (around zero field) and the fixed layerare separated. The stress dependence of the fixed layer is practicallyzero, while the free layer has a stress response similar to a single Nilayer. Tensile stress makes the magnetisation rotate away from thestress direction, while compressive stress tends to pull it towards thestress direction. FIG. 22 b shows the hard axis of the free layer as themiddle part of the figure (between −4 kA/m and 4 kA/m), and of the fixedlayer as the parts outside this region. Again the fixed layer maintainsits magnetic characteristics under stress, while the free layer respondsas described above. It has to be noticed that the a non stress hard andsoft free layer, due to the coupling with the fixed layer, is presence.FIG. 23 shows the results measured at a Ni-based tunnel junctiondeposited on a bimorphous substrate. The Ni dictates the magnetisationof the free layer, hence giving it its voltage sensitivity.

Furthermore, experiments were carried out at lower frequencies, whichshowed absorption of a shear SAW in a magnetic layer, manifested by aphase dependence of the SAW on an applied magnetic field (FIG. 24). Asshown in FIG. 24 phase of a shear wave depends on the applied magneticfield. The amount of change is up to 4 degrees. The different curves arefor different biasing fields. The magnetic field is measured by a Hallprobe. All measured loops are between −100 and 100 Gauss. Notice thepresence of the hysteresis of the magnetic material in the SAW behavior.This means the strains interact. All results were demonstrated on singlelayers, as well as magnetic components.

Simulations have been performed to study the effect on a magneticelement of the combination of an additional magnetic field at resonancefrequency and a SAW field at double the resonance frequency, accordingto the present invention. These simulations show that the SAW canchange, e.g. amplify, the amplitude and the damping of the oscillationsthat are driven by a magnetic field. Theoretically, alternatively, thesaw field can have a frequency substantially equal to an even multipleof the ferromagnetic resonance frequency. Illustrations are given inFIG. 25 where it is shown that by application of the SAW field theoscillation amplitude of the magnetic material, seen as an absorptionpeak, is enhanced. The oscillation amplitude is plotted for differentvalues of the magnetic field amplitude (legend) and in function of theratio of the SAW amplitude versus magnetic field amplitude with a fixedphase relationship between the SAW and the magnetic field. At zero SAWamplitude, the excited resonance is equal to the classical ferromagneticresonance peak. At higher SAW values, the oscillation amplitude isincreased due to the application of the SAW field. The increase can bechanged by tuning the value of the phase relationship between the SAWand the magnetic field.

Another illustration is given in FIG. 26, where the damping of magneticmotion towards the equilibrium state, represented by the Gilbert dampingparameter α, is actively controlled by the amplitude of the SAW.

The SAW can thus change the resonant properties of the ferromagneticmaterial and therefore the SAW can also alter the switching behaviour ofthe magnetic element. In this way the SAW helps in controlling thedamping process in the ferromagnetic material. Control over the dampingparameter is important e.g. to optimise the switching speed of themagnetic element. For too low damping values, the magnetic element canswitch very fast but never stops ringing, i.e. precessing, whereas fortoo high values of the damping, the magnetic element will switch tooslow.

It is to be understood that although preferred embodiments, specificconstructions and configurations, as well as materials, have beendiscussed herein for devices, systems and methods according to thepresent invention, various changes or modifications in form and detailmay be made without departing from the scope and spirit of thisinvention.

1. A method comprising: providing a device that includes: at least oneferromagnetic element having a ferromagnetic resonance frequency ν_(FMR)and being capable of magneto-elastic energy conversion; and a layercomprising piezoelectric material, the layer being adapted fortransporting a surface acoustic wave having a frequency ν_(SAW); whereinthe layer comprising piezoelectric material is in contact with the atleast one ferromagnetic element and the surface acoustic wave frequencyν_(SAW) is substantially equal to the ferromagnetic resonance frequencyν_(FMR) or an integer multiple of the ferromagnetic resonance frequencyν_(FMR); generating a surface acoustic wave in the layer comprisingpiezoelectric material, such that the surface acoustic wave interactswith the ferromagnetic element to influence a magnetization of theferromagnetic element; dynamically measuring a variation of acharacteristic parameter influenced by the ferromagnetic element;deriving, from the variation of the characteristic parameter, acorresponding value of a measured quantity.
 2. A method according toclaim 1, further comprising: allowing the ferromagnetic element tointeract with an environment for which an environmental quantity is tobe measured; wherein the deriving of the measured quantity includesderiving the environmental quantity from the variation of thecharacteristic parameter.
 3. A method according to claim 2, wherein thephysical quantity of the ferromagnetic element is a magneto resistanceof the ferromagnetic element.
 4. A method according to claim 2, whereinthe deriving from the variation of the characteristic parameter acorresponding value of the physical quantity comprises: deriving fromthe dynamic measurement a degree of anisotropy of the at least oneferromagnetic element; and deriving from the degree of anisotropy acorresponding value of the physical quantity.
 5. A method according toclaim 2, wherein the variation in the characteristic parameterinfluenced by the at least one ferromagnetic element is induced by themagnetization or magnetization direction of the ferromagnetic element.6. A method according to claim 2, further comprising applying anadditional magnetic field at the ferromagnetic resonance frequency or aninteger multiple of the ferromagnetic resonance frequency ν_(FMR).
 7. Amethod according to claim 6, wherein the surface acoustic wave has afrequency ν_(SAW) substantially equal to the ferromagnetic resonancefrequency ν_(FMR) or an integer multiple of the ferromagnetic resonancefrequency ν_(FMR).
 8. A method according to claim 6, wherein the integermultiple is an even integer multiple.
 9. A method according to claim 1,wherein the device includes a plurality of said ferromagnetic elementsin contact with the piezoelectric layer, further comprising: allowingthe plurality of ferromagnetic elements to interact with an environmentfor which a magnetic image is to be created; and wherein: the generatinga surface acoustic wave includes generating a surface acoustic wave thatinteracts with the plurality of ferromagnetic elements so as toinfluence a magnetization state of the ferromagnetic elements; thedynamic measuring includes dynamically measuring, for each of theplurality of ordered ferromagnetic elements, a variation ofcharacteristic parameters influenced by the ferromagnetic elements; andthe deriving includes deriving, from the variation of the characteristicparameters, a corresponding value of a physical quantity for each of theplurality of ordered ferromagnetic elements.
 10. A method according toclaim 9, wherein the physical quantity for each of the ferromagneticelements is a magneto resistance of the ferromagnetic elements.
 11. Amethod according to claim 9, wherein the allowing the plurality ofordered ferromagnetic elements to interact with an environment and thegenerating a surface acoustic wave is performed one time for allferromagnetic elements in parallel and wherein the dynamically measuringthe variation and the deriving a corresponding value is performed on aper ferromagnetic element basis.
 12. A method according to claim 9,further comprising applying an additional magnetic field at theferromagnetic resonance frequency or an integer multiple of theferromagnetic resonance frequency ν_(FMR).
 13. A method according toclaim 12, wherein the surface acoustic wave has a frequency ν_(SAW)substantially equal to the ferromagnetic resonance frequency ν_(FMR) oran integer multiple of the ferromagnetic resonance frequency ν_(FMR).14. A method according to claim 12, wherein the integer multiple is aneven integer multiple.
 15. A method according to claim 1, wherein themeasured quantity is a readout-value of the ferromagnetic element, andwherein: the generating of the surface acoustic wave is such that aprecessional movement of a magnetization in the ferromagnetic element isachieved and such that a corresponding magnetization state of the atleast one ferromagnetic element is not switched.
 16. A method accordingto claim 15, wherein the characteristic parameter influenced by theferromagnetic element is a magneto resistance of the ferromagneticelement.
 17. A method according to claim 15, wherein the deriving fromthe variation of the characteristic parameter the read-out valuecomprises: deriving a phase difference between an input signal appliedto a surface acoustic wave generating means and an output signalobtained from the dynamic measurement of the characteristic parameter;and deriving from the phase difference the read-out value.
 18. A methodaccording to claim 15, wherein the read-out value corresponds with adistinct number of specific values.
 19. A method according to claim 15,further comprising applying an additional magnetic field at theferromagnetic resonance frequency or an integer multiple of theferromagnetic resonance frequency ν_(FMR).
 20. A method according toclaim 19, wherein the surface acoustic wave has a frequency ν_(SAW)substantially equal to the ferromagnetic resonance frequency ν_(FMR) oran integer multiple of the ferromagnetic resonance frequency ν_(FMR).21. A method according to claim 19, wherein the integer multiple is aneven integer multiple.
 22. A method according to claim 15, furthercomprising switching the ferromagnetic element by a method thatincludes: generating a surface acoustic wave that operates to achieve aprecessional movement of a magnetization in the ferromagnetic element;and orienting a corresponding magnetization state of the ferromagneticelement.
 23. A method for switching at least one ferromagnetic element,comprising: generating a surface acoustic wave in a layer comprisingpiezoelectric material, the layer being in contact with the at least oneferromagnetic element, the generating a surface acoustic wave being forachieving a precessional movement of a magnetization in theferromagnetic element; and orienting a corresponding magnetization stateof the ferromagnetic element.
 24. A method according to claim 23,wherein the orienting the magnetization state of the ferromagneticelement is performed by generating a ferromagnetic element specificadditional field.
 25. A method according to claim 23, wherein thesurface acoustic wave is one of a Rayleigh wave with an angle between aneasy axis of the ferromagnetic element and a direction of effectivefield of 90° during a first half period of the Rayleigh wave and a shearwave with the angle between the direction of an easy axis of theferromagnetic element and the direction of the effective magnetic fieldgenerated greater than 45°.
 26. A method according to claim 23, furthercomprising applying an additional magnetic field at the ferromagneticresonance frequency or an integer multiple of the ferromagneticresonance frequency ν_(FMR).
 27. A method according to claim 26, whereinthe surface acoustic wave has a frequency ν_(SAW) substantially equal tothe ferromagnetic resonance frequency ν_(FMR) or an integer multiple ofthe ferromagnetic resonance frequency ν_(FMR).
 28. A method according toclaim 26, wherein the integer multiple is an even integer multiple. 29.A method according to claim 1, further comprising: monitoring absorptionof a surface acoustic wave by the ferromagnetic element; deriving fromthe absorption a difference between a working frequency of the surfaceacoustic wave and a ferromagnetic resonance frequency of theferromagnetic element; and tuning the working frequency of the surfaceacoustic wave towards the ferromagnetic resonance frequency.
 30. Amethod according to claim 29, wherein the tuning of the workingfrequency of the surface acoustic wave generating means towards theferromagnetic resonance frequency is tuning the working frequency to afrequency slightly different from the ferromagnetic resonance frequency.31. A method according to claim 30, wherein the frequency correspondswith the absorption of the surface acoustic wave by the ferromagneticelement within 1% and 99% of the absorption of the surface acoustic waveby the ferromagnetic element at the ferromagnetic resonance frequency.